1. Field of the Invention
The present invention relates to ferrous metal surface corrosion inhibition, and particularly to a novel rinse step following a conventional phosphating treatment.
2. Description of the Prior Art
In metal finishing treatments for iron and steel, a chemical conversion coating is commonly deposited. Such coatings are of the iron phosphate of zinc phosphate type.
The coatings are applied to improve adhesion of paint and to enhance the overall corrosion resistance. The non-painted coatings do not offer much protection from corrosion. In order to compensate for this deficiency, the coatings are rinsed after the phosphating stage in what is referred to as an inhibiting seal or rinse.
The inhibiting rinse most commonly employed in the art is composed of chromic acid and water or chromic acid, phosphoric acid and water. The latter is commonly referred to as the modified chromic acid rinse.
These substances work extremely well and are commonplace in the metal finishing industry. However, they are not without certain disadvantages. First, the chromic acid by nature of its color has a tendency to stain and "bleed" through light-colored paints. Frequently, it is necessary to utilize a subsequent water rinse or reduce the amount of inhibiting chromic acid based seal in order to eliminate the problem. Secondly, it is of interest for environmental reasons to reduce or eliminate materials which are toxic in nature.
Thus the toxic nature of chromium rinses requires either the use of expensive disposal systems for removal of toxic chromium radicals, or the use of an alternative rinse system which has a simpler disposal problem. Accordingly, considerable effort has been expended to develop a satisfactory inhibiting seal or rinse which provides an effluent easily brought within standards of the EPA.
U.S. Pat. No. 3,966,502 utilizes a zirconium rinse, and the patentee suggests that the zirconium run-off can easily be recovered. Various other patentees have suggested other alternative rinses, all of which have advantages and disadvantages. Obviously, the most desirable rinse would be sufficiently compatible with the environment that no effluent treatment would be needed. The present invention provides such a rinse.